The speeds of metal-oxide-semiconductor (MOS) transistors are closely related to the drive currents of the MOS transistors, which drive currents are further closely related to the mobility of charges. For example, NMOS transistors have high drive currents when the electron mobility in their channel regions is high, while PMOS transistors have high drive currents when the hole mobility in their channel regions is high.
Germanium is a commonly known semiconductor material. The electron mobility and hole mobility of germanium are greater than that of silicon, which is the most commonly used semiconductor material in the formation of integrated circuits. Hence, germanium is an excellent material for forming integrated circuits. However, in the past, silicon gained more popularity over germanium since its oxide (silicon oxide) is readily usable in the gate dielectric of MOS transistors. The gate dielectrics of the MOS transistors can be conveniently formed by thermal oxidation of silicon substrates. The oxide of germanium, on the other hand, is soluble in water, and hence is not suitable for the formation of gate dielectrics.
With the use of high-k dielectric materials in the gate dielectrics of MOS transistors, however, the convenience provided by the silicon oxide is no longer a big advantage, and hence germanium is reexamined for use in integrated circuits.
In addition to germanium, compound semiconductor materials of group III and group V elements (referred to as III-V compound semiconductors hereinafter) are also good candidates for forming NMOS devices for their high electron mobility.
A challenge faced by the semiconductor industry is that although the MOS transistors formed on germanium and III-V compound semiconductors have high drive currents, the leakage currents of these MOS transistors are also high. This is partially caused by the low bandgap and the high dielectric constants of germanium and the III-V compound semiconductors. For example, FIG. 1 illustrates the comparison of bandgaps and dielectric constants of germanium and some commonly used III-V compound semiconductors, and some other semiconductor materials. FIG. 1 reveals that the bandgaps of germanium and some commonly used III-V compound semiconductors are small. Therefore, the respective MOS transistors suffer from band-to-band leakage currents between their gates and source/drain regions. The high dielectric constants of these materials further worsen the leakage currents. As a result, the on/off current ratios (Ion/Ioff) of these MOS transistors are relatively low.